This invention relates to a semiconductor power circuit breaker, and, more particularly, it relates to a device having a structure which prevents the occurrence of secondary breakdown at low threshold voltages.
The operation of a semiconductor power circuit breaker such as a switching transistor or a Gate Turn Off (GTO) thyristor is characterized by two operating states. In the current conducting state or on state, a residual voltage generally corresponding to the saturation voltage and being on the order of magnitude of one volt or less is applied to the power circuit breaker. In the off state or blocking state, the entire operating voltage is applied to the semiconductor power circuit breaker. When switching from the on state to the off state and vice versa, the power circuit breaker passes through areas in the family of characteristics called "safe operating areas" (SOA). These areas indicate the current/voltage values which the power circuit breaker must not exceed as a function of the operating temperature as a parameter. These safe operating areas are defined for both the conducting state and non-conducting state. The safe operating area for the non-conducting state is the more important one because the non-conducting state is the more critical one. One reason is that the off behavior of the semiconductor substantially determines the capacity of the equipment in which the device is installed or may be associated in operation.
The safe operating area or region of a semiconductor power circuit breaker for the cutoff process, i.e. when the emitter-base-pn junction is biased in the blocking or reverse direction, called RBSOA (Reverse Bias SOA) is shown diagrammatically in FIG. 1. It is designated as region A for a conventional semiconductor power circuit breaker in dc operation and may be extended into region B for pulse operation. For both cases, the maximum operating voltage is U.sub.CEO, i.e. the maximum emitter-collector voltage with open base terminal. This voltage must not be exceeded if the breaker is not to fail because of "secondary breakdown".
FIG. 2 illustrates this process. Therein is shown a semiconductor power circuit breaker with a semiconductor body. It has a first zone 1 which is n-doped, for example. Abutting zone 1 is a second zone 2 of the opposite conduction type with higher doping than zone 1. Abutting the second zone 2 is a third zone 3 of the first conduction type with higher doping than the second zone 2. The side of the first zone 1 facing away from the second zone 2 is abutted by a fourth zone 6 which is n-doped for a switching transistor and higher doped than zone 1. For a GTO thyristor, this zone is p-doped and has a higher doping concentration than zone 1.
It should also be pointed out that power transistors of this general configuration are also known to include multiple emitter regions such as disclosed in a commonly assigned application now matured into U.S. Pat. No. 4,626,886.
In the breaker's cut-in-state, the emitter electrode 5 is subjected to a negative potential, the anode electrode 7 zero potential, and the gate electrode is maintained at a positive potential. Zone 1 is then flooded with charge carriers and the breaker is conducting. For blocking, the gate electrode 4 is reverse biased with respect to the emitter electrode 5. Then the charge carriers stored mainly in zone 1 drain toward electrodes 4 and 7, respectively. In FIG. 1, the path of the positive charge carriers to the electrode 4 is marked by arrows 1. These carriers follow the path produced by the direction of the field intensity in the direction to the emitter zone 3 and then drain radially outward to the electrodes 4. This generates an emitter bias which is highest in the center of the emitter 3. If it exceeds approximately 0.7 volts in the center of the emitter 3, the emitter will start to emit carriers at this point. Once stared, this process can yet spread and grow by carrier multiplication until the breaker is destroyed by local overheating. This phenomemon is called secondary breakdown. Therefore, a conventional switching transistor may be operated below the voltage at which carrier multiplication does not yet occur. This voltage is the voltage U.sub.CEO shown in FIG. 1.